Digital three-dimensional object manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional object printing is an additive process in which one or more ejector heads deposit material to build up a part. Material is typically deposited in discrete quantities in a controlled manner to form layers which collectively form the part. The initial layer of material is deposited onto a substrate, and subsequent layers are deposited on top of previous layers. The substrate is supported on a platform that can be moved relative to the ejection heads so each layer can be printed; either the substrate is moved via operation of actuators operatively connected to the platform, or the ejector heads are moved via operation of actuators operatively connected to the ejector heads. Three-dimensional object printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
In many three-dimensional object printing systems, a partially printed part is subjected to a leveling process after each layer of material is deposited. The leveling process ensures that each layer is a controlled thickness, and that the subsequent layer has a flat surface to be formed upon. By performing this leveling process between each successive layer, higher quality parts are manufactured within narrower tolerances.
In some three-dimensional object printing systems, a leveling roller flattens the upper surface of the part after each successive layer of material is deposited. FIG. 6 shows a prior art three-dimensional object printing system 100 having a conveyer 104 and a leveling roller 108. The conveyer 104 has a substantially planar surface 112 upon which printed parts, such as the partially formed part 116, are built. The conveyer 104 is configured to convey the part 116 in a conveying direction X that is parallel to the surface 112 of the conveyer 104. The roller 108 is arranged above the surface 112 of the conveyer 104 in a vertical direction Y that is normal to the surface 112 of the conveyer 104. The roller 108 is cylindrical about a longitudinal axis that extends in a lateral direction Z, which is parallel to the surface 112 of the conveyer 104 and orthogonal to the conveying direction X.
After each successive layer of material is deposited, the conveyer 104 conveys the part 116 in the conveying direction X. The roller 108 is adjusted to an appropriate distance from the surface 112 of the conveyer 104. The conveyer 104 feeds the part 116 between the conveyer 104 and the roller 108 to flatten an upper surface 120 of the part 116 that is opposite a bottom surface of the part 116 that sits upon the surface 112 of the conveyer 104.
The printing system 100 is designed to handle parts, such as the part 116, up to 20 inches wide in the lateral direction Z, but the roller 108 is intended to only remove about 3 microns of material from the upper surface 120 of the part 116. This constraint imposes costly manufacturing tolerances for the roller 108. For example, the roller 108 can be twenty inches long and two inches in diameter. This relatively large roller must be manufactured with tight tolerances for cylindricity. Particularly, the roller must be manufactured with tight tolerances for straightness and roundness. As used herein “straightness” refers to the variability of the roller's diameter across its length. As used herein “roundness” refers to the variability in diameter that depends on the angle from which the diameter measured. A roller with perfect roundness has precisely the same diameter when measured from all angles. Conversely, a roller having imperfect roundness has variances in diameter that depend on the angle from which it is measured. This variance in diameter at different angles is referred to as “run-out.”
FIG. 7 shows a side view of the printing system 100 with a roller 108 having imperfect roundness, or run-out. A circular outline 204 shows an ideal roundness of the roller 108. As can be seen, portions of the roller 108 extend beyond the circular outline 204. The particular run-out of the roller 108 varies with each roller that is manufactured. Accordingly, the roller 108 is incapable of truly flattening the upper surface 120 of the part 116 unless the run-out of the roller is eliminated, but significant manufacturing costs must be incurred for the elimination of the run-out.
FIG. 8A and FIG. 8B show the effect of the run-out of the roller 108 on the leveling process. As the roller 108 moves over the upper surface 120 of the part 116, the longitudinal axis of the roller 108 maintains a fixed distance from the conveyer 104. However, because the diameter of the roller 108 varies, a ripple is produced in the upper surface 120 of the part 116 as the roller 108 moves across the part 116, as seen in FIG. 8B. Accordingly, the run-out of the roller 108 adversely impacts the leveling process.
In current printing systems, such as the printing system 100, the rollers 108 are ground to very tight tolerances on the order of one micron to minimize the effect of the run-out. The roller 108 can be manufactured at reasonable costs within one micron of variability in its straightness. However, manufacturing the roller 108 with tighter tolerances for roundness comes at great expense. What is needed is a low cost leveling assembly that can accurately flatten the upper surface of a part as the part is formed without requiring a large roller manufactured within such tight tolerances for roundness.